The present invention concerns a device for the maintenance and control of the oscillations of a balance wheel using an electro-dynamic maintenance system.
Systems are known in which the balance wheel is controlled, when affected by a serious departure from isochronism, by synchronisation with a reference frequency derived from a quartz oscillator. The principle consists in measuring the phase difference between the oscillation of the balance wheel and the reference signal and in then providing the balance wheel, on the basis of this information, with a signal such that the phase difference tends to diminish with time.
The amplitude of oscillation of the balance wheel may vary continuously between two limits or have two discrete values, a low value (approximately 150.degree.) and a high value (approximately 250.degree.).
In known systems, the balance wheel obtains a variable energy maintenance level by modifying continuously or discretely, the width and/or amplitude of the maintenance pulses.
However, the balance wheel amplitudes are very badly defined, for they depend on the losses, hence on the quality factor (Q) of the balance wheel. Now, this factor may vary, particularly as a function of the position of a watch, and after a lapse of time. In extreme cases, which are often found with small balance wheels, the high-energy amplitude with the watch in a vertical position may be less than the low energy amplitude with the watch in a horizontal position, and then synchronisation becomes impossible.
Even if a range of synchronisation exists, it is always greatly reduced by any increase in friction and, consequently, the elimination of a phase displacement takes longer.